The present invention relates to fiber of high strength and high modulus of elasticity comprising a polyoxymethylene copolymer, and a process for producing the same. More particularly, by use of a polyoxymethylene copolymer with a crystallization rate controlled appropriately, improvement of a process for producing fiber, or combination thereof, occurrence of in-fibril voids during stretching is inhibited, whereby polyoxymethylene fiber having high strength and high modulus of elasticity can be obtained.
A polyoxymethylene copolymer having an oxymethylene group as main repeating units is excellent in mechanical properties such as rigidity and strength, chemical resistance, solvent resistance, electric properties and the like, and has a high crystallization rate, and therefore it is a material very suitable for general molding processes such as injection molding, and is widely used for working parts of automobiles and electric appliances taking advantage of its various properties and molding processability.
On the other hand, owing to these mechanical properties, chemical resistance, solvent resistance, electric properties and the like, fiber comprising the polyoxymethylene (co)polymer is expected to be applied to a variety of products such as concrete reinforcing fiber and various kinds of cross-meshed filters.
However, because of the high crystallization rate, the polyoxymethylene (co)polymer has a problem such that in-fibril voids occur and thus fiber is easily cut during melt spinning of fiber and stretching process and as a result, productivity cannot be improved, and fiber having high strength cannot be obtained.
In addition, in JP-A 60-183122, JP-A 61-54921 or the like, a process for producing polyacetal having high strength and high modulus of elasticity by highly stretching a polyacetal (polyoxymethylene) molded article by making the polyacetal molded article pass through a pressurized fluid is disclosed, and filaments and the like are illustrated as obtained stretched articles, but this process is lacking in productivity as a process for producing polyacetal (polyoxymethylene) fiber, and has a problem such that very fine and uniform fiber cannot be obtained.
As described above, the fiber comprising a polyoxymethylene (co)polymer has excellent properties and is expected to be used based on the properties, but is not yet in the actual use due to the above problems, and alleviation of the problems has been desired.
The present invention is to solve the problems such as those described above and provide fiber of high strength and high modulus of elasticity comprising polyoxymethylene, and a process for producing the same with high production efficiency.
As a result of vigorous studies for achieving the above purpose, the inventors found that a crystallization rate of polyoxymethylene to use, and heating conditions of fiber spun from a nozzle and taken off in production of fiber are very important factors, and conducted further detailed studies, resulting in completion of the present invention.
That is, the present invention is firstly directed to polyoxymethylene fiber comprising a polyoxymethylene copolymer that has a half-crystallization time of at least 30 sec when the polyoxymethylene copolymer is cooled from a molten state at 200xc2x0 C. to 150xc2x0 C. at a cooling rate of 80xc2x0 C./min and maintained constantly at the temperature of 150xc2x0 C. (hereinafter referred to as the first invention sometimes).
The present invention is secondly directed to a process for producing the polyoxymethylene fiber characterized by taking off fibrous matter spun from a spinning nozzle of melt spinning apparatus while heating in an ambient temperature of from 140 to 250xc2x0 C., when the polyoxymethylene copolymer is subjected to melt spinning to produce the polyoxymethylene fiber (hereinafter referred to as the second invention sometimes).
The present invention is further directed to a fiber use of the above polyoxymethylene copolymer or a polyoxymethylene copolymer produced by the above process.